The Haskell Platform is a comprehensive, robust development for programming
in Haskell, which not only makes it trivial to get up and running with a full
Haskell development environment, but also provides a comprehensive, standard
base for commercial and open source Haskell development that maximises
interoperability and stability of your code.

This requires ghc.

This is intended to be installed on a system that does not already have any
haskell-related components (other than ghc, of course) installed, so for best
results (and chance of success), you should remove any of those before you 
try to build haskell-platform.
